Management of uplink resources in multi-carrier CDMA system

ABSTRACT

The method an apparatus described herein manages uplink resources to increase spectral efficiency and system capacity. According to one embodiment of the present invention, a base station may be assigned two or more downlink carriers for downlink transmission and two or more corresponding uplink carriers. In a multi-carrier mode, the base station may transmit signals on two or more downlink carriers to the same mobile terminal, and receive signals from the mobile terminal on one of the paired uplink terminals. The uplink carriers can be operated at different interference levels and the uplink traffic can be divided between the available uplink carriers based on the type of traffic and/or data transmission parameters. The mobile terminals may also be allowed to switch between the uplink carriers to improve overall efficiency.

BACKGROUND

The present invention relates generally to multi-carrier communicationsystems and, more particularly, to management of uplink resources in amulti-carrier communication system.

Enhanced uplink, also known as High Speed Uplink Packet Access (HSUPA)was introduced in Release 6 of the Wideband Co-Division Multiple Access(WCDMA) standard to provide higher data rates on the uplink. HSUPAsupports data rates of up to 11.52 megabits per second in the uplinkusing higher order modulation, fast power control, fast scheduling, andfast hybrid ARQ (HARQ) with soft combining. Two new physical uplinkchannels were added to the WCDMA standards to support HSUPA: theenhanced dedicated physical data channel (E-DPDCH) and the enhanceddedicated physical control channel (E-DPCCH). The E-DPDCH is the uplinkchannel used to carry user data bits from the mobile terminal to thebase station, referred to in the standard as an enhanced NodeB (eNodeB).The E-DPCCH carries control information necessary to enable the basestation to demodulate and decode the E-DPDCH.

Conventional WCDMA systems operate with a single uplink carrier. Release8 of the WCDMA standard will allow transmission from the base station tothe mobile terminals on two adjacent carriers. On the uplink, the mobileterminals will still use a single carrier for uplink transmissions.However, the mobile terminals may be allowed to switch between twodifferent uplink carriers that are paired with the two downlinkcarriers.

Realizing the high data rates that are supported by HSUPA has beenchallenging. When a user terminal transmits at a high data rate on theuplink carrier, a high signal-to-interference plus noise ratio (SINR) atthe receiver is needed in order to demodulate and decode thetransmission. This means that the user terminals' receive power at thebase station must be high, which will create interference for otherusers (e.g., voice users or low rate data users), as well as importantcontrol channels. These other users will then need to increase theirtransmit power to avoid degradation, thus further increasing theinterference levels at the receiver. When the interference levels becometoo high, the system becomes unstable.

Therefore, improvements in managing uplink resources are needed in orderto manage the interference generated by high data rate users.

SUMMARY

The present invention provides a method an apparatus for managing uplinkresources to increase spectral efficiency and system capacity. Accordingto one embodiment of the present invention, a base station may beassigned two or more downlink carriers for downlink transmission and twoor more corresponding uplink carriers. In a multi-carrier mode, the basestation may transmit signals on two or more downlink carriers to thesame mobile terminal, and receive signals from the mobile terminal onone of the paired uplink terminals. However, the mobile terminaltransmits signals on the uplink using only one of the paired uplinkcarriers.

The uplink carriers can be operated at different interference levels andthe uplink traffic can be divided between the available uplink carriersbased on the type of traffic and/or data transmission parameters. As oneexample, one uplink carrier may be used to carry voice, low-rate data,and control channels, while a second carrier may be used to carryhigh-rate data. By segregating different types of traffic on differentcarriers, the low-rate data, control channels, and other traffic carriedon the anchor carriers are protected from excessive levels ofinterference attributable to the high-rate data transmissions.

The mobile terminals may also be allowed to switch between the uplinkcarriers to improve overall efficiency. For example, a mobile terminalmay be allowed to switch from a first uplink carrier operated at a lowinterference level to a second uplink carrier operated at a highinterference level depending on a data transmission rate and/or bufferlevel. The mobile terminal may also switch uplink carriers based on ahappiness indication reflecting the buffer level of the mobile terminal.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an exemplary multicarrier communication system.

FIG. 2 illustrates an exemplary spectrum allocation for a base stationin a multicarrier communication system.

FIG. 3 illustrates an exemplary method implemented by a base station ina multicarrier communication system for selecting a carrier for uplinktransmissions.

FIG. 4 illustrates an exemplary method implemented by a base station ina multicarrier communication system for controlling uplink transmissionsfrom a plurality of mobile terminals.

FIG. 5 illustrates an exemplary base station for a multicarriercommunication system.

DETAILED DESCRIPTION

Referring now to the drawings, FIG. 1 illustrates a user terminal 100 ina mobile communication network 10. The user terminal 100 may comprise,for example, a cellular telephone, personal digital assistant, smartphone, laptop computer, handheld computer, or other device with wirelesscommunication capabilities. User terminal 100 communicates with a basestation 20 in a serving cell or sector 12 of the mobile communicationnetwork 10. The user terminal 100 receives signals from the base station20 on one or more downlink (DL) channels and transmits signals to thebase station 20 on one or more uplink (UL) channels.

For illustrative purposes, an exemplary embodiment of the presentinvention will be described in the context of a Wideband Code DivisionMultiple Access (WCDMA) system. Those skilled in the art willappreciate, however, that the present invention is more generallyapplicable to other wireless communication systems, including Long TermEvolution (LTE) and WiMAX (IEEE 802.16) systems.

Enhanced Uplink in WCDMA, also referred to as High Speed Uplink PacketAccess, provides high speed uplink access the mobile terminals 100served by the base station 20. The mobile terminals 100 transit data tothe base station 20 on a transport channel referred to as the EnhancedDedicated Channel (E-DCH). As the name implies, the E-DCH is a dedicatedchannel. At any given time, a mobile terminal 100 may transmit on one ormore E-DPDCHs, which is the physical data channel associated with theE-DCHs. A scheduler at the base station 20 coordinates transmissions bythe mobile terminals 100 on the uplink. The mobile terminals 100 reportbuffer levels, power headroom, QoS requirements, and other schedulinginformation to the base station 20 in a scheduling request. Based on theinstantaneous interferences levels at the receiver and the schedulinginformation received from the mobile terminals 100, the base station 20determines which mobile terminals 100 are allowed to transmit and atwhat rates. The base station 20 transmits scheduling grants to thescheduled mobile terminals 100 to indicate when and at what rate themobile terminals 100 are allowed to transmit. The scheduling granttypically specifies a ratio of E-DPDCCH-to-pilot power ratio allowed forthe scheduled mobile terminal 100, and the mobile terminal 100 isallowed to select any transport block size (data rate) so long as thespecified power ratio is not exceeded. In general, a higher power ratiocorresponds with a higher data rate.

Conventional WCDMA systems operate with a single uplink carrier. Release8 of the WCDMA standard allows transmission from the base station 20 tothe mobile terminals 100 on two adjacent carriers. On the uplink, themobile terminals 100 will still use a single carrier for uplinktransmissions. However, the mobile terminals 100 may be allowed toswitch between two different uplink carriers that are paired with thetwo downlink carriers.

FIG. 2 illustrates an exemplary spectrum allocation for multi-carrierHSPA. Carriers D1 and D2 are allocated for the downlink, andcorresponding paired uplink carriers U1 and U2 are allocated for theuplink. In a multi-carrier mode, base station 20 uses carriers D1 and D2for downlink transmissions to mobile terminal 100. In contrast, themobile terminal 100 may use either one, but not both, of the uplinkcarriers U1 and U2 for uplink transmission. Mobile terminal 100 may alsoswitch between uplink carriers U1 and U2 in different time periodsdepending on the data transmission rate of the mobile terminal 100and/or other transmission parameters. There is a current 3GPP work itemwith the goal of introducing transmission on two adjacent carriers fromthe mobile terminals to the base station in future releases of the WCDMAstandard. Still, it may be more beneficial to have the mobile terminalstransmit on one carrier at a time only.

According to one embodiment of the present invention, the uplinkcarriers U1 and U2 are operated at different interference levels and theuplink traffic is divided between the two available uplink carriers U1and U2 to improve spectral efficiency on the uplink. More specifically,one uplink carrier may be designated as an anchor carrier and operatedat a relatively low interference level (e.g., 5-8 dB noise rise). Theother uplink carrier, referred to herein as a supplemental carrier ornon-anchor carrier, may be operated at a relatively high interferencelevel (e.g., >15 dB noise rise) compared to the anchor carrier. Theuplink carriers U1 and U2 may be used for different types of traffic.For example, the anchor carrier may be used to carry voice, low-ratedata, delay-sensitive data, and control channels. The supplementalcarrier may be used to carry high-rate data and other types oftransmissions that generate high levels of interference. In oneexemplary embodiment, the anchor carrier may include control channelsfor all traffic channels on both the anchor and non-anchor carriers. Ifa mobile terminal 100 is transmitting on the supplemental carrier, itcannot transmit an associated control channel on the anchor carrier,because the mobile terminal 100 can only transmit on one carrier at anygiven time. However, other mobile terminals can transmit controlchannels on the anchor carrier. By segregating different types oftraffic on different carriers, the low-rate data, control channels, andother traffic carried on the anchor carriers is protected from excessivelevels of interference attributable to the high-rate data transmissions.

At the base station 20, serving a cell 12 may schedule the mobileterminals 100 within the cell 12 to transmit on either the anchorcarrier or supplemental carrier, depending on its transmissionrequirements. For example, it may be more efficient to schedule themobile terminals having high data rate transmissions to transmit on thesupplemental carrier using time-division multiplexing (TDM) because TDMprovides better orthogonality between users. For low data ratetransmissions, however, the base station 20 may schedule the mobileterminals to transmit on the anchor carrier using code-divisionmultiplexing (CDM) because CDM has better trunking efficiency. Table 1summarizes the differences between an anchor carrier and a supplementalcarrier for one embodiment of the present invention.

TABLE 1 Anchor Carriers Supplemental Carriers Interference Level Low(5-8 dB noise rise) High (>15 dB noise rise) Traffic Voice, control,low-rate High-rate data and/or delay insensitive data Scheduling CDM TDM

The carrier assignment can be signaled to the mobile terminals 100 in ascheduling grant transmitted on a downlink control channel. For example,the carrier assignment for a mobile terminal 100 can be transmitted tothe mobile terminal 100 as part of an absolute grant transmitted on theEnhanced Absolute Grant Channel (E-AGCH). As is known in the art, thescheduler can update the serving grant of a mobile terminal 100 bysending an absolute grant. The absolute grant may be modified to includea field specifying the carrier to which the grant applies. In responseto the absolute grant, the mobile terminal 100 may switch to the carrierspecified in the absolute grant if a switch is necessary.

The decision to assign a mobile terminal 100 to the anchor carrier orsupplemental carrier is made by a scheduler at the base station 20 basedon scheduling information received from the mobile terminal 100. Aspreviously noted, the scheduling information may include informationsuch as the buffer level, available transmission power, QoSrequirements, etc. Such information may be transmitted in-band on theEnhanced Dedicated Channel (E-DCH). Additionally, mobile terminal 100may transmit a happiness indication, also referred to as a “happy bit,”on the Enhanced, Dedicated Physical Control Channel (E-DPCCH). Generallyspeaking, the happiness indication comprises a single bit that indicateswhether the mobile terminal 100 is capable of transmitting on the E-DCHat a data rate greater than what is currently allowed by the servinggrant. When the mobile terminal 100 has available power to transmit at ahigher data rate than allowed by the serving grant, and the number ofbits in the buffer would require more than a predetermined number ofTTIs to transmit, the mobile terminal 100 sets the happy bit to a firstpredetermined value to indicate that it is “not happy.” “Not happy”means that the mobile terminal 100 would like to transmit at a higherdata rate. Otherwise, the mobile terminal 100 sets the happy bit to asecond predetermined value to indicate that it is “happy.” It may benoted that the happy bit is only transmitted in conjunction with anon-going data transmission because the E-DPCCH is only transmittedtogether with the E-DPDCH.

In some embodiments of the invention, the happiness indication may beused to facilitate carrier switching. For example, assume that mobileterminal 100 is currently transmitting data on an anchor carrier. Whenthe happiness indication is set to “happy,” the scheduler at the basestation 20 may continue to schedule the mobile terminal 100 on theanchor carrier. On the other hand, when the happiness indication is setto “not happy,” the scheduler at the base station 20 can switch themobile terminal 100 to the supplemental carrier to enable higher datatransmission rates without generating interference for other users onthe anchor carrier.

In some embodiments, more than two levels of happiness may be definedand the happiness indication may include more than one bit. Thedifferent levels of happiness may be related to the number of TTIs thatthe mobile terminal 100 would require to empty its transmit buffer underthe current serving grant. For example, a tri-level happiness indicationcould be defined using two thresholds, denoted herein as T1 and T2,where T2 is less than T1. If the number of TTIs needed to empty thetransmit buffer is greater than T1, the mobile terminal 100 may set thehappiness indication to “not happy.” If the number of TTIs required toempty the transmit buffer is greater than T2 but less than T1, themobile terminal 100 may set the happiness indication to “slightlyhappy.” Finally, if the number of TTIs required to empty the transmitbuffer is less than T2, the mobile terminal 100 can set the happinessindication to “happy.”

In some embodiments, several levels of “happiness” could be defined byimposing multiple thresholds denoted as T_(x) representing the number ofTTI's that the mobile terminal 100 needs to empty its transmit buffer atthe current grant rate. For example, if the mobile terminal 100 canempty its buffer in N TTI's and N>T₁ . . . , the happy bit can be set to“Not Happy”. If T₂≦N≦T₁, then the happy bit can be set to “SlightlyHappy”. If N<T₂, the happy bit can be set to “Happy”. “Not Happy” couldmean switching to or continuing on a supplemental carrier; “SlightlyHappy” could mean switching to an anchor carrier if on supplementalcarrier; and “Happy” could mean continuing on an anchor carrier, ordiscontinuing transmission temporarily.

When the system uses more than two types of carriers, the happinessindication can be used to bias the scheduler to select a particularcarrier. In one exemplary embodiment, the system may be configured witha low data rate carrier, a medium data rate carrier, and a high datarate carrier. In this example, the happiness indication can be used tobias the scheduler towards a higher data rate carrier as the happinesslevel decreases. The degree of the bias may depend on the level ofhappiness. Regardless, some embodiments of the invention is use “HappyBit(s)” in the scheduler to determine which carrier or carrier type andperhaps how many carriers the UE need to be scheduled.

FIG. 3 illustrates an exemplary method 150 implemented by a base station20 for controlling transmissions from a plurality of mobile terminals100 on the uplink. Base station 20 controls the interference level on afirst uplink carrier to meet a first interference target (block 152).The first interference target may, for example, comprise a low level ofinterference. The base station 20 also controls the interference levelon a second uplink carrier to meet a second interference target higherthan the first interference target (block 154). A scheduler at the basestation 20 determines the data transmission requirements for a pluralityof mobile terminals (block 156). Based on the data transmissionrequirements, the scheduler assigns each of the mobile terminals 100 toone of the uplink carriers (block 158). For example, the scheduler atbase station 20 may assign low data rate mobile terminals 100 to thefirst uplink carrier, and high data rate mobile terminals to the seconduplink carrier. Also, QoS requirements may be taken into account incarrier selection. For example, mobile terminals 100 with low delaytolerance may be assigned to the first carrier.

FIG. 4 illustrates a method 200 implemented by a scheduler in basestation 20 of selecting a carrier for uplink transmissions from a mobileterminal 100. The method 200 begins when the scheduler assigns a mobileterminal 100 to a first uplink carrier and begins receivingtransmissions from the mobile terminal 100 (block 202). When the mobileterminal 100 is transmitting data, the mobile terminal 100 may alsotransmit a happiness indication on the E-DPCCH. The scheduler at thebase station 20 receives the happiness indications from the mobileterminal 100 (block 204). The scheduler reassigns the mobile terminal100 to the second uplink carrier depending, at least in part, on thehappiness indication (block 206). For example, when mobile terminal 100is transmitting on an anchor carrier for low data rate users, thescheduler may reassign the mobile terminal 100 to a second carrier ifthe happiness indication transmitted by the mobile terminal 100indicates that the mobile terminal 100 is “not happy.” In otherembodiments, the happiness indication may be used to bias schedulingdecisions toward the higher data rate carrier as the happiness leveldecreases.

FIG. 5 illustrates an exemplary base station 20 according to oneembodiment of the invention. Base station 20 comprises a transceiver 24coupled to one or more antennas 22 and a baseband processor 26.Transceiver 24 comprises a transmitter for transmitting signals tomobile terminals 100, and a receiver for receiving signals from themobile terminals 100. Baseband processor 26 comprises one or moreprocessors, microcontrollers, hardware, or a combination thereof. Thebaseband processor 26 processes signals transmitted and received by thetransceiver 24. For example, the baseband processor 26 may performcoding/decoding, modulation/demodulation, interleaving/deinterleaving,and other channel coding operations. Baseband processor 26 includes ascheduler 28 for scheduling the uplink transmissions from the mobileterminals 100. The scheduler 28 includes logic for performing the methodshown in FIGS. 3 and 4.

The present invention may, of course, be carried out in other ways thanthose specifically set forth herein without departing from essentialcharacteristics of the invention. The present embodiments are to beconsidered in all respects as illustrative and not restrictive, and allchanges coming within the meaning and equivalency range of the appendedclaims are intended to be embraced therein.

What is claimed is:
 1. A method of controlling transmissions from aplurality of mobile terminals on an uplink carrier, said methodcomprising: controlling an interference level on a first uplink carrierto meet a first interference target; controlling an interference levelon a non-first uplink carrier to meet a second interference targethigher than the first interference target; determining individual datatransmission requirements for the plurality of mobile terminals;assigning each of said mobile terminals to transmit user data on eitherthe first uplink carrier or the non-first uplink carrier based on saiddata transmission requirements; scheduling users on said first uplinkcarrier using code-division multiplexing; and scheduling users on saidnon-first uplink carrier using time-division multiplexing.
 2. The methodof claim 1 wherein the step of determining individual data transmissionrequirements for the plurality of mobile terminals comprises determiningdata rate requirements for said mobile terminals, and wherein saidmobile terminals are assigned to transmit user data on either said firstuplink carrier or said non-first uplink carrier based on said data raterequirements.
 3. The method of claim 2 wherein mobile terminalsrequiring a high data transmission rate are assigned to the non-firstuplink carrier, and mobile terminals requiring a low data transmissionrate are assigned to the first uplink carrier.
 4. The method of claim 1wherein the step of determining individual data transmissionrequirements for the plurality of mobile terminals comprises determiningdelay sensitivity for said mobile terminals, and wherein said mobileterminals are assigned to transmit user data on either said first uplinkcarrier or said non-first uplink carrier based on said delaysensitivity.
 5. The method of claim 4 wherein mobile terminals with ahigh delay sensitivity are assigned to the first uplink carrier, andmobile terminals with a low delay sensitivity are assigned to thenon-first uplink carrier.
 6. A base station in a multicarriercommunication system, said base station comprising: a transceiver toreceive user data transmitted by a plurality of mobile terminals on twoor more uplink carriers; and a scheduler to schedule transmissions fromsaid plurality of mobile terminals on said uplink carriers, saidscheduler configured to: control an interference level on a first uplinkcarrier to meet a first interference target; control an interferencelevel on a non-first uplink carrier to meet a second interference targethigher than the first interference target; determine individual datatransmission requirements for the plurality of mobile terminals; assignthe mobile terminals to transmit user data on either the first uplinkcarrier or the non-first uplink carrier based on said data transmissionrequirements; schedule users on said first uplink carrier usingcode-division multiplexing; and schedule users on said non-first uplinkcarrier using time-division multiplexing.
 7. The base station of claim 6wherein the scheduler is configured to determine data rate requirementsfor said mobile terminals, and assign said mobile terminals to eithersaid first uplink carrier or said non-first uplink carrier based on saiddata rate requirements.
 8. The base station of claim 7 wherein thescheduler is configured to assign mobile terminals requiring a high datatransmission rate to the non-first uplink carrier, and to assign mobileterminals requiring a low data transmission rate to the first uplinkcarrier.
 9. The base station of claim 6 wherein the scheduler isconfigured to determine a delay sensitivity for said mobile terminals,and to assign said mobile terminals to either said first uplink carrieror said non-first uplink carrier based on said delay sensitivity. 10.The base station of claim 9 wherein the scheduler is configured toassign mobile terminals with a high delay sensitivity to the firstuplink carrier, and to assign mobile terminals with a low delaysensitivity to the non-first uplink carrier.